This invention relates generally to high speed printers, such as impact line printers, of the type arranged to drive print hammers by means of solenoids, and more particularly, the present invention relates to a device for accurately controlling the energizing interval of respective solenoids of such a printer.
As is well known, there are two sorts or types of impact line printers, namely, back impact type and front impact type, and the present invention may be applicable to both of these types.
High speed impact printers comprise a type carrier in the form of a drum or a looped belt for carrying a plurality of types which will be selectively impacted by the print hammers. Although the present invention may be adapted to both types of printers having a type drum or type belt, the following description will be made in connection with a printer having a type drum. High speed impact printers are further divided into two sorts. In a first sort, each print hammer is independently driven to impact each type on the type drum or belt one after another in a given sequence from the first place to the last place in a print line. In a second sort, one or more print hammers is/are driven simultaneously to impact one or more types on the type drum or belt at the same time. In both the first and second sorts, scanning is performed to check the relationship between print data indicative of each character to be printed in a single line and character codes indicative of characters on the type drum or belt. In the first sort, each print hammer is driven one after another to complete printing of a single line, while in the second sort, one or more print hammers is/are driven simultaneously once a scanning. As will be understood from the following description, the present invention may be adapted to these first and second sorts of high speed impact printers.
In back impact type line printers, a type carrier, such as a type belt or a type drum, arranged to pass a printing position, and a plurality of print hammers arranged along the print positions, facing the type carrier, are provided so that printing is effected by copying respective characters of type faces on a print sheet passing between the type carrier and the print hammer by means of an ink ribbon or the like. Each of the print hammers must be driven for a given period of time for performing desirable printing and for preventing ghosting, smudging or misregistration of printed characters.
The plurality of the print hammers are respectively driven by solenoids which are arranged to be energized by switching elements, such as power transistors. Therefore, in order that each print hammer is driven for a given period of time, it is necessary to continuously operate a corresponding switching element for a predetermined period of time.
To this end, up to this time, a monostable multivibrator has been used generally for energizing each solenoid for a given period of time. As is well known, the output pulse width of a monostable multivibrator is determined by the time constant which is defined by the resistance of a resistor and the capacitance of a capacitor. However, the resistance and the capacitance have relatively wide variation ranges. Accordingly, the above-mentioned energizing interval can be accurately controlled only when the resistance of a variable resistor is manually adjusted. However, since the number of circuits, whose time constant must be adjusted in the above-mentioned manner, generally corresponds to the number of places of characters in one print line, the number is very large, for instance 132. For this reason, it has been time consuming to effect such adjustment of time constants of monostable multivibrators. Furthermore, capacitors have a large variation range against temperature changes, and therefore, the pulse widths of such monostable multivibrators have been affected by temperature changes.
According to another already proposed method for controlling the energizing interval to a given value, scanning interval or type travelling time, i.e. the interval between adjacent characters, is utilized, where the scanning interval is a period of time for which the contents of a line print buffer storing print data of one line is compared with the contents of a character code generator generating codes of characters of the type carrier which faces each printing place.
In this method, however, there is a problem that the structure of a control circuit is complex in the case that the energizing interval of each solenoid is not an integral multiple of the scanning interval or the interval between characters, while such a problem does not occur if the energizing interval is set to an integral multiple of the same. Furthermore, since the scanning interval as well as the interval between adjacent characters changes in accordance with the speed variation of the type carrier, poor printing, such as partial printing of a character or misregistration of a character cannot be avoided when the moving speed of the type carrier changes.